pubsys, QUICKSTART: upload OVAs via vSphere's content library instead of directly to the inventory

**What I'd like:**
To better support downstream users of the vmware variant of Bottlerocket, we should align with EKS-A's vSphere provisioning process as much as possible. Their process involves importing OVAs via vSphere's content library through `govc library.import` instead of directly to the inventory through `govc import.ova`.

The vSphere content library apparently does additional checks against the OVA compared to just importing the OVA.

See https://github.com/bottlerocket-os/bottlerocket/blob/f825677d128e0931b57efe08dfc0424968e055fc/tools/pubsys/src/vmware/govc.rs#L78-L86

and https://github.com/bottlerocket-os/bottlerocket/blob/develop/QUICKSTART-VMWARE.md#upload-the-ova

**Any alternatives you've considered:**
Keeping it the same.

Start with tools/pubsys/src/vmware/govc.rs at the referenced import logic and compare it with the upload section in QUICKSTART-VMWARE.md. Review EKS-A's vSphere provisioning process and determine the changes needed to use govc library.import with the content library; update the implementation and quickstart so both describe the same workflow.

Written by the indexing model from the issue text.